Process for adjusting the ph of a cellulosic pulp, production of pulp or paper from cellulosic pulp and use of (bi)carbonate removal for reducing the consumption of acid

ABSTRACT

A process for adjusting the pH of a cellulosic pulp in a pulp and/or paper mill, and to the production of pulp, paper or board from so treated cellulosic pulp is disclosed. In the process, the pH of an alkaline cellulosic pulp is adjusted at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment. Between the first and the second carbon dioxide induced pH adjustment there is provided a (bi)carbonate removal step, which reduces the buffering capacity of the pulp. Also provided for is the use of (bi)carbonate removal for reducing the consumption of acid in the treatment of a cellulosic pulp.

The present application claims priority from Finland Patent Application No. 20095026, filed Jan. 14, 2009.

FIELD OF THE INVENTION

The invention relates to a process for adjusting the pH of a cellulosic pulp in a pulp and/or paper mill, and to the production of pulp, paper or board from so treated cellulosic pulp. The invention also relates to the use of (bi)carbonate removal for reducing the consumption of acid in the treatment of a cellulosic pulp. Carbon dioxide is used in the invention to adjust the pH of the pulp.

BACKGROUND OF THE INVENTION

Cellulosic pulp is produced in a pulp mill by chemical or mechanical treatment of wood chips. Cellulosic pulp may also be provided by repulping of recycled fibers.

Although there are a number of different processes available, the most frequently used process for producing chemical pulp is the so called Kraft process. In this process, wood chips are cooked in the presence of sodium hydroxide and sodium sulfide. A highly alkaline mixture of digested pulp and cooking liquor (black liquor) is obtained. The black liquor is removed and the pulp is typically washed, optionally delignified with oxygen, washed, filtered and washed again.

Digested and optionally oxygen delignified pulp is still fairly dark in color and for producing bright grades of products it needs to be bleached. In the bleaching plant, the pulp is typically bleached with acidic and or alkaline bleaching agents and extracted and washed between treatments. Bleaching chemicals include chlorine, hypo chlorite, chlorine dioxide, oxygen, hydrogen peroxide, ozone, enzymes, dithionite, etc. As the pH changes from alkaline to acidic and back to alkaline or neutral, huge amounts of acids and bases are consumed.

Washing is an important operation in removing from the fibers chemicals and components which have been detached from the fibers in the previous digestion, bleaching and/or extraction operation. Various ways for improving the washing of the fibers in the pulp mill have been created. One such prior art washing process, which has been much used in the art is described in EP-B1-0 296 198 (AGA). The document relates to improved washing of an alkaline pulp. The washing-out of COD contributing substances is improved when the pH is reduced by gaseous carbon dioxide added to the pulp and/or to the water used for washing the pulp. A description of mill trials using the carbon dioxide washing technique of EP 0 296 198 was made by G. Östberg in a presentation called “Use of Carbon Dioxide in the Production of Sulphate Pulp” at the 5^(th) international conf. on new available techniques, Stockholm, 4-7 Jun. 1996.

Carbon dioxide (CO₂) has also been used in various positions of the pulping and papermaking process to adjust the pH of the pulp. Carbon dioxide dissolves in water and provides carbonic acid (H₂CO₃), which in turn dissociates and creates carbonate (CO₃ ²⁻) and bicarbonate (HCO₃ ⁻) ions in the pulp. Use of carbon dioxide for adjusting the pH of an alkaline pulp has been described, for instance, in EP-B1-0 281 273 (BOC).

Enzyme treatment is a fairly new form of pulp treatment and new enzymes are continuously being produced for purifying and bleaching the pulp. A common enzyme is based on xylanase, which catalyzes xylan hydrolysis and promotes removal of lignin. Enzyme treatment is typically performed at a pH around neutral (pH 6-9) and it is typically followed by an acidic bleaching sequence including one or more chlorine dioxide stages at a pH of about 2 to 4. U.S. Pat. No. 5,645,686 (Solvay) describes a process for treating a chemical pulp by a xylanase stage followed by a peroxyacid bleaching stage. US Patent Application 2005/0150619 (logen Bioproducts) describes the use of xylanase after a chlorine dioxide bleaching stage.

Chelating agents are sometimes used in the treatment of pulp for removing heavy metals from the pulp. WO 1996/011298 (Sunds Defibrator) describes a method of chlorine free bleaching of pulp where the metals in the pulp are removed by means of a chelating agent. The pulp is treated with the chelating agent at a pH 2-7 for a period of 5 minutes to 2 hours and then washed.

The pH adjustment of the pulp to suit various alkaline, neutral and acidic treatments in pulp and paper mills is traditionally done by using strong bases such as sodium hydroxide and strong acids such as sulfuric acid. Carbon dioxide can be used to provide carbonic acid, which is a weak acid and which therefore enables a more exact control of the pH.

According to G. Östberg in the above mentioned presentation, carbon dioxide is suitable in pulp mills for adjusting alkaline liquors in the neutral pH range. Examples of positions, where carbon dioxide can be used to adjust the pH are, according to Östberg, before a chelating stage or an enzyme stage. Dissolved in water, carbon dioxide produces carbonic acid but also bicarbonate and carbonate ions, which exert a buffering effect on the pulp. In the regulation of the pH and in the washing of a pulp, such as described by Östberg, the (bi)carbonate ions provide a positive effect in the system. However, the present inventor has noticed that when the pH of the buffered pulp needs to be adjusted after the carbon dioxide induced pH adjustment, the very same (bi)carbonate ions will resist pH changes and increase the amount of acid or base needed to change the pH.

In treatment sequences, wherein the pH of the pulp needs to be changed from alkaline or neutral to acidic or vice versa, the buffering ions created by the carbon dioxide, will negatively influence the subsequent pH adjustment.

There is a need in the art for methods which allow the use of carbon dioxide as a pH controlling agent without adversely affecting the attaining of a low pH for a subsequent acidic step. It is an object of the invention to satisfy that need.

All documents cited in this text as well as each document or reference cited in each of the so cited documents, and all manufacturer's literature, specifications, instructions, product data sheets, material data sheets, and the like, as to the products and processes mentioned in this text, are hereby expressly incorporated herein by reference.

SUMMARY OF THE INVENTION

In the invention, a (bi)carbonate removal step is provided for reducing the buffering effect of a carbon dioxide induced pH adjustment. By reducing the buffering effect, a subsequent pH adjustment to a low pH value is facilitated. Thus, the invention has the effect of reducing the amount of strong acid needed to reach a desired low pH value after carbon dioxide addition. When the strong acid comprises a bleaching chemical, the invention has the effect of reducing the amount of bleaching chemical needed.

The invention relates to a process for adjusting the pH of a cellulosic pulp. The process includes the steps of: providing an alkaline cellulosic pulp, wherein said pulp has been produced by a chemical, mechanical and/or repulping treatment and has been subsequently subjected to washing and optional oxygen delignification; adjusting the pH of the alkaline cellulosic pulp so obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment; and providing a (bi)carbonate removal step between the first and the second carbon dioxide induced pH adjustment.

The carbon dioxide induced pH adjustments may be performed by adding carbon dioxide directly into the pulp. Alternatively, the carbon dioxide can be added to water, which is then added to the pulp. Such water is typically water used for diluting and/or washing the pulp.

In a preferred embodiment of the invention, the alkaline pulp has been produced by chemical digestion, typically in a Kraft process.

In one embodiment of the invention, the first carbon dioxide induced pH adjustment is performed in connection with pulp washing. The second carbon dioxide induced pH adjustment is performed prior to the neutral treatment to adjust the pH of the pulp to suit the neutral treatment in question. The second pH adjustment is typically performed as a step separate from washing. The aim of the second pH adjustment is to provide the desired pH value for the subsequent neutral treatment and this is typically not the same as the aim of a pulp wash.

The pH adjustments with carbon dioxide create carbonate and bicarbonate ions in the aqueous pulp suspension. These ions have a buffering effect in the neutral pH range at about pH 6-8, which means that they resist pH adjustments to a lower pH value. According to the invention, (bi)carbonate ions are removed from the pulp after the first carbon dioxide induced pH adjustment and prior to the second carbon dioxide induced pH adjustment.

In one embodiment of the invention, the (bi)carbonate removal step comprises using water for displacing and/or washing (bi)carbonate ions from the pulp after the first carbon dioxide induced pH adjustment.

The (bi)carbonate removal step of the invention may also comprise dewatering and/or compression of the pH-adjusted pulp. Dewatering and compression are typically followed by subsequent dilution of the pulp with water.

Adding carbon dioxide after the (bi)carbonate removal step to adjust the pH of the pulp a second time creates new (bi)carbonate ions in the pulp. However, the total count of (bi)carbonate ions in the twice pH adjusted pulp is less than it would have been without the intermediate removal step. Since the amount of buffering (bi)carbonate ions in the pulp is reduced, the subsequent pH reduction with a strong acid to suit the acidic treatment of the pulp requires less acid to overcome the buffering effect of the (bi)carbonates.

The invention also relates to a process for the production of pulp, board or paper from cellulosic pulp which has been treated in a pulp and/or paper mill according to the inventive pH-adjustment scheme.

The invention also relates to the use of (bi)carbonate removal for reducing the consumption of acid in the acidification of an alkaline cellulosic pulp.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an embodiment of the invention in a Kraft pulp mill.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to processes which utilize carbon dioxide for adjusting the pH of a pulp. In one embodiment, the invention relates to treatment sequences in the fiber line of a pulp mill wherein the pH needs to be changed from alkaline to neutral and then to acidic. The acidic treatment in the fiber line typically comprises bleaching of the pulp.

By providing a pH adjustment with carbon dioxide divided into two separate carbon dioxide additions and by providing a (bi)carbonate removal step between them, the invention enables the mill to take advantage of the benefits of the carbon dioxide pH adjustment without adversely affecting the subsequent acidification. Removing (bi)carbonates after the first carbon dioxide induced pH adjustment reduces the amount of strong acid which is needed in the acidification. This provides clear technical benefits and reduced chemical consumption.

The strong acids which are typically used in a pulp or paper mill are sulfate-rich acids such as sulfuric acid and waste acid from the chlorine dioxide production. Reducing the amount of acid has a marked effect on the purity of the process. The pulp typically contains minor amounts of barium, which will form harmful scaling on the equipment under sulfate-rich conditions. In a pulp mill the use of sulfuric acid contributes to the over-all sodium/sulfur balance of the mill. Reducing the amount of the strong acid may improve the Na/S balance of the mill.

Dividing the carbon dioxide induced pH adjustment into two separate ones may also save carbon dioxide compared to the attaining of the same pH with one carbon dioxide addition. The (bi)carbonate removal step also removes organic components of the pulp which might otherwise consume bleaching chemicals. This improves the process and enables the use of less bleaching chemicals. Moreover, when the strong acid comprises a bleaching chemical, such as chlorine dioxide, the invention enables considerable savings in bleaching chemicals.

The pulp which is to be treated according to the invention is an alkaline pulp in a pulp and/or paper mill. A chemical pulp is typically produced by chemical digestion of wood chips, such as in a Kraft cooking process. After digestion, the cooking liquor is removed and the pulp is washed. The washing is typically followed by oxygen delignification to free a greater part of the lignin. The oxygen delignification is followed by washing and/or filtering processes, where after the pulp is ready for bleaching or other purification processes. Alternatively, the oxygen delignification is omitted and the pulp is instead subjected to one or more additional washings. The pulp is then dewatered and dried or sent to the paper mill for production into paper or board.

Other pulps, including mechanical and semi-mechanical pulps as well as reclaimed fibers also undergo alkaline, neutral and acidic treatments and the invention is suitable for improving the pH adjustment of such pulps also.

After the washing and the optional oxygen delignification, a Kraft pulp has a pH which varies depending on the preceding treatment. The pH after digestion in a Kraft process is about 12-14. Oxygen delignification is performed at a pH of 12 to 14 and the pulp coming from the delignification typically has a pH of 10 to 13. Washing typically reduces the pH slightly even if no carbon dioxide is added to the washing stage.

The initial pH of the alkaline cellulosic pulp which is subjected to the pH adjustments according to the invention is typically between pH 8 and 13 and most typically between 9.5 and 11. The neutral treatment for which the subsequent pH adjustment with carbon dioxide is made, is typically performed at a pH between 6 and 9 and most typically at 6.5 to 8. This pH is the one to be reached with the second carbon dioxide induced pH adjustment, while the first carbon dioxide induced pH adjustment adjusts the pH to between pH 7 and 12 and most typically to pH 8.5 to 11.

In order to provide a precise control of the pH for the neutral treatment, the carbon dioxide of the second pH adjustment is preferably introduced directly into the pulp. The carbon dioxide may, however, also be introduced into water used for diluting the pulp.

The subsequent acidic treatment of the pulp is one which requires a lower pH than the neutral treatment. The acidic treatment is typically performed in the range of pH 2 to 7. Sometimes pH values even lower than 2 are desired. In case the neutral treatment is performed between about pH 8 and 9, the acidic treatment may be performed at pH 6 to 7. However, the acidic treatment is typically one which is performed at pH below 6 and most typically at pH 2-4. The pH adjustment to suit the acidic treatment is performed with a strong acid, i.e. an acid which is stronger than carbonic acid.

Various ions enter the pulp with the wood or are present in the chemicals added to the process in the various treatments in a pulp and/or paper mill. Carbonates and bicarbonates are typically created in the cooking and in the oxygen delignification of the pulp. A part of these (bi)carbonates probably leave the pulp with the other impurities removed in the washing operations to which the cooked pulp is subjected. The amount of (bi)carbonate in a Kraft pulp may be as high as 10 kg or more calculated as kg CO₂/ton of dry pulp.

Carbon dioxide used for washing or pH adjustment of the pulp creates (bi)carbonate ions in the pulp. The invention is mainly involved with removing the (bi)carbonates which enter the pulp when carbon dioxide is added to the pulp or to the aqueous medium of the pulp for adjusting the pH. However, it is evident that the (bi)carbonate removal step of the invention will also affect (bi)carbonates created or added in other earlier unit operations. In a pulp with a high content of (bi)carbonate ions derived from other positions than the carbon dioxide addition, the invention will have a double beneficial influence.

The carbon dioxide is most easily added in gaseous form as a gas containing or consisting of carbon dioxide. It should be noted that the carbon dioxide need not necessarily be gaseous. The invention works also with liquid or even solid carbon dioxide. The carbon dioxide gas may be pure carbon dioxide gas or a gas containing carbon dioxide, such as flue gas. Air also includes carbon dioxide, but the amount of carbon dioxide (0.03%) in air is generally too low for industrial purposes.

The amount of carbon dioxide added in any single or combined carbon dioxide induced pH adjustment depends on the conditions of the pulp, such as the pH of the alkaline pulp and the desired pH of the neutral treatment, the presence or absence of disturbing chemicals such as other buffering compounds, the amount of carbon dioxide that can be introduced and dissolved into the pulp, etc. The skilled person will be able to select the correct amount of carbon dioxide by monitoring the pH of the pulp. Typically, the amount of carbon dioxide introduced in any one addition point ranges between 0.5 and 15 kg per ton of dry pulp. Generally an amount of 1 to 10 kg carbon dioxide is sufficient for providing the desired effect.

When carbon dioxide enters an aqueous medium, the following reactions take place:

CO₂+H₂O→H₂CO₃

H₂CO₃→H⁺+HCO₃ ⁻

HCO₃ ⁻→H⁺+CO₃ ²⁻

The created hydrogen ions lower the pH of the pulp. The hydrogen ions will be present in the aqueous medium of the pulp suspension and they will also enter the fibers themselves. It is believed that some of the hydrogen ions attach to negatively charged phenolic groups within the fibers and oust cations like sodium and possibly also calcium from the fibers. It is believed that this mechanism is at least one of those responsible for the improved washing result provided by the lowered pH. The mobile anions, i.e. the bicarbonates and carbonates will mostly be present in the aqueous medium surrounding the fibers. As the fibers comprise immobilized negatively charged groups, these will exert a repelling force on the negatively charged (bi)carbonate ions. Thus, there is a strongly reduced tendency for (bi)carbonate ions compared to the tendency of hydrogen ions to enter the fiber water.

Without wishing to be bound by any theory, it is believed that the different behavior of the hydrogen ions and the (bi)carbonate ions, respectively, relative to the fibers is the one which makes it possible to selectively remove (bi)carbonate ions from the pulp without at the same time removing the corresponding amount of pH influencing hydrogen ions.

In the invention the pH of an alkaline pulp is reduced with carbon dioxide to a value between pH 6 and 9 for a neutral treatment of the pulp. In the prior art such a pH adjustment would have been made simply by adding carbon dioxide to the pulp until the desired pH was reached. However, in the invention, the carbon dioxide is added in two separate steps with a (bi)carbonate removal step in between. The (bi)carbonate removal reduces the buffering capacity (M-alkalinity) of the pulp.

The first carbon dioxide induced pH adjustment of the invention may be performed in any desired manner and it need not, and typically will not, result in the exact pH desired for the neutral treatment. When the second pH adjustment with carbon dioxide is performed after the (bi)carbonate removal step according the invention, this second pH adjustment will be monitored so as to provide the desired pH for the neutral treatment. The second carbon dioxide addition will create new (bi)carbonate ions in the pulp. However, the sum of (bi)carbonates in the twice pH adjusted pulp will be significantly less than if the pH adjustment had been done as in the prior art by a single pH adjustment with carbon dioxide.

In an embodiment of the invention, the first carbon dioxide induced pH adjustment is performed in connection with a pulp washing step. This procedure will have the dual benefit of improving the washing of the alkaline pulp and adjusting the pH down to a value closer to the desired pH of the neutral treatment.

The second carbon dioxide induced pH adjustment is typically performed just prior to the neutral treatment. The carbon dioxide addition is typically monitored to adjust the pH of the pulp with carbon dioxide to suit the neutral treatment.

The neutral treatment of the pulp may be any treatment which is performed on the pulp in the neutral pH range between an alkaline and an acidic treatment. Such treatments include a variety of treatments at different points of the pulp and papermaking process, such as an enzyme stage in a pulp mill, a flotation stage in a deinking plant and a refining stage in a paper mill.

The subsequent pH adjustment with a strong acid is performed after said neutral treatment and before said acidic treatment. The pH of the pulp is adjusted with the strong acid to a pH which is suitable for the acidic treatment. When the acidic treatment is a bleaching stage such as a chlorine dioxide stage, the strong acid is typically selected from the group consisting of sulfuric acid, sulfurous acid, bisulfite, chlorine dioxide and waste acid from chlorine dioxide production. When the acidic treatment is a rosin sizing process, the strong acid comprises alum. Thus, the strong acid is selected by those skilled in the art so as to suit the respective acidic treatment in question.

In many processes there will be none or only very minor unit operations between the neutral treatment and the acidic treatment. In a typical pulp bleaching process, there will be no washing stage between the neutral and the acidic treatments. However, according to an embodiment of the invention, there is introduced a second (bi)carbonate removal step after the neutral treatment and before the addition of the strong acid. This will further remove buffering (bi)carbonate ions and will facilitate the attaining of the desired low pH.

It is also possible to provide the neutral treatment and the carbon dioxide induced pH reduction and (bi)carbonate removal in the middle of a bleaching sequence such as after an initial acidic bleach and alkaline extraction.

As discussed above, the anions of the carbonic acid created in the first carbon dioxide induced pH adjustment are removed selectively compared to the hydrogen ions of the acid. It is evident that the removal step will not generally remove all (bi)carbonate ions nor will all of the hydrogen ions remain in the aqueous medium. However, due to the different behavior of the said ions, there is a significant difference in their removal rate.

There are many ways in which the (bi)carbonate ions can be selectively removed from the pulp without at the same time removing the corresponding hydrogen ions, i.e. without significantly increasing the pH of the pulp. The (bi)carbonate removal of the invention may comprise one, two or more individual steps, which in turn comprise one or more individual modes of (bi)carbonate removal which combine to provide the desired (bi)carbonate reduction. The typical modes of (bi)carbonate removal according to the invention comprise washing, filtering, displacing, dewatering, compression and combinations thereof.

The (bi)carbonate removal may also comprise a combination of dilution and dewatering. Dilution causes diffusion of any (bi)carbonate ions from the fiber water to the aqueous medium surrounding the fibers. Dilution without subsequent removal of water from the diluted pulp does not reduce the amount of (bi)carbonates in the pulp.

In certain situations, (bi)carbonate removal may be accomplished by precipitation of calcium carbonate by means which are well known to those skilled in the art. However, care should be taken to avoid dissolving the precipitated solid at a later stage.

In an embodiment of the invention, the (bi)carbonate removal step comprises using water for displacing and/or washing (bi)carbonate ions from the pulp. One preferred (bi)carbonate removal step comprises subjecting a low consistency pulp (e.g. 1-6%) to a filtering action and displacing the (bi)carbonate containing aqueous medium of the pulp by shower water applied on the filter.

A typical mode of removing the (bi)carbonate ions is to subject the (bi)carbonate containing pulp to a washing step. In the washing, the (bi)carbonate containing pulp is treated with washing water and the water is thereafter removed. The water removal carries away a large proportion of the (bi)carbonate ions. Pulp washing steps are typically performed in washing stages, which comprise one or more washing steps. Thus a washer used in a washing stage may have several washing steps in sequence with a filtration or dewatering step between the washing steps. The pH of the (bi)carbonate removing wash water is preferably selected so that it does not increase the pH of the pulp.

Washing and filtering replaces most of the water surrounding the fibers in a pulp suspension with new water. Thus, a bicarbonate removal step according to the invention comprising washing and filtering typically removes between 10 and 70% of the (bi)carbonate ions in a pulp. Dewatering and compression may remove between 10 and 75% of the (bi)carbonate ions.

The pulp may also be compressed after a filtering or washing step to remove as much as possible of the (bi)carbonate ions. A combination of washing and compression will typically remove from 25 to 90% or more of the (bi)carbonate containing water. After compression, the resulting higher consistency pulp is typically diluted with water to allow pumping and further treatment of the pulp.

It is preferred to perform the (bi)carbonate removal step(s) in such a manner that at least 25% of the (bi)carbonate ions created by the first carbon dioxide induced pH adjustment is removed. Most preferably, the (bi)carbonate removal step(s) remove up to 90% or more of the (bi)carbonates in the pulp. In a typical (bi)carbonate removal process, several (bi)carbonate removal modes are used in combination. For example, a wash press can be used to remove 70 to 85% of the created (bi)carbonates. Extra efficient washing/filtering/compression steps may remove as much as 95 to 100% of the (bi)carbonates.

The (bi)carbonate removal percentage in any position depends on the mode(s) of removal selected, on the operation of the equipment in question, on the amount of water added and removed, on the ionic strength of the pulp, etc. The person skilled in the art will appreciate that the above expressed percentages are only approximate and that the removal can be improved or reduced in the normal optimization of the process.

The (bi)carbonate removal percentage in a removal step is calculated as a reduction in (bi)carbonate content (measured as kg CO₂ per metric ton of dry pulp) of a pulp exiting the removal step as compared to the (bi)carbonate content of the pulp entering the said (bi)carbonate removal step. In case the pulp entering or exiting the (bi)carbonate removal step is diluted with water, the measurement counts as one made before any dilution has taken place with carbonate containing water.

The water used for displacing, washing and/or dilution typically has a lower content of (bi)carbonate ions than the pulp being subjected to the (bi)carbonate removal step.

When the alkaline pulp comprises a Kraft pulp which has been subjected to oxygen delignification, the (bi)carbonate removal step typically comprises at least one step of washing the pulp.

The water used for washing, displacing or diluting the pulp in the (bi)carbonate removal step is typically fresh water, condensate, clear filtrate or mixtures of such clean waters, or it may comprise washing water/filtrate from downstream pulp washing. In case the (bi)carbonate removal step is followed by a washing step, the filtrate from said washing step is preferably used. However, water from a washing stage downstream of the neutral treatment may also be used in the (bi)carbonate removal step. Mixtures of clean circulating waters may also be used.

In an embodiment of the invention, the (bi)carbonate removal step comprises at least two modes of (bi)carbonate removal. Such modes may be selected from displacing, washing, filtering, dewatering and compression of the pulp.

In an embodiment of the invention, the neutral treatment of the pulp comprises an enzyme treatment or a chelating step after the second pH adjustment with carbon dioxide. The acidic treatment typically comprises an acidic bleaching treatment with a bleaching chemical selected from chlorine dioxide, ozone, peroxyacid and dithionite.

In a preferred embodiment of the invention, the neutral treatment comprises an enzyme treatment, wherein the enzyme typically comprises xylanase, and the acidic treatment comprises chlorine dioxide bleaching. The enzyme treatment is preferably conducted before any bleaching sequences, although it is also possible to perform the enzyme treatment after an initial chlorine dioxide and extraction sequence.

The acidic bleaching treatment typically includes a sequence of stages. An initial chlorine dioxide stage (D₀) is typically followed by an alkaline extraction stage (E) and a second chlorine dioxide stage (D₁). The sequence may also comprise an ozone stage (O) and/or a peroxyacid stage (Pa). Sometimes alkaline bleaching stages such as hydrogen peroxide stages (P) are included in the sequences. Mechanical pulps are typically bleached with dithionite and/or hydrogen peroxide and they may be subjected to chelating stage(s).

Alkaline, neutral and acidic treatment stages are also encountered in other positions of the pulp and/or paper making process.

When recovered paper is converted into papermaking pulp, the paper is first broken down in a pulping process which is performed at high pH, typically pH 9 to 10, at an elevated temperature. The fibers are usually screened and dewatered then subjected to flotation in a neutral pH range between 6.5 and 8.5 to remove ink particles. Then the cleaned pulp may be treated with acidic agents, e.g. in bleaching or sizing operations.

When the pH adjustment of the invention is utilized for recycled fibers, the first carbon dioxide induced pH adjustment is typically provided before or in an initial flotation stage. The first pH adjustment may be performed in a washing step, as a direct pH adjustment operation or by way of a flotation gas. The second pH adjustment is preferably performed before a subsequent post-flotation stage. Between the flotation stages, the pulp is washed and dewatered to remove (bi)carbonates and ink particles.

By performing the carbon dioxide induced pH adjustment for the flotation according to the invention in two separate steps, the cleaned pulp will have a lower buffering capacity than if the pH adjustment had been made in one step as in the prior art. A subsequent acidic treatment according to the invention will consequently consume less strong acid.

Carbon dioxide addition and (bi)carbonate removal according to the invention may also be performed after a flotation stage or in the absence of flotation to adjust the pH of an alkaline pulp to suit a neutral papermaking process. The consumption of any acidic chemicals such as bleaching and fixative agents can be reduced in this manner.

It should be noted that recycled fibers containing high amounts of calcium carbonate should normally not be subjected to highly acidic treatments since calcium carbonate easily dissolves during most acidic treatments. However, the (bi)carbonate ions created and remaining in the pulp after the second carbon dioxide induced pH adjustment of the invention will help to retain calcium carbonate in solid form even under mildly acidic conditions.

When an alkaline pulp is to be refined at a neutral pH and sized at an acidic pH such as described in EP-B1 0 281 273 (BOC), the desired sizing pH may be obtained with considerable savings in strong acid (sulfuric acid and alum) if the pH is adjusted twice with carbon dioxide and (bi)carbonates are removed between the carbon dioxide induced pH adjustments in accordance with the invention.

The pulp which has been treated in accordance with any of the embodiments of the invention comprises raw material in the production of paper, board or dry pulp. The invention also encompassed a process for producing pulp, board or paper from the so treated cellulosic pulp.

Accordingly, the invention also relates to a process for the production of pulp, board or paper from cellulosic pulp, which is characterized in

-   -   a. providing an alkaline cellulosic pulp in a pulp and/or paper         mill, wherein said pulp has been produced by a chemical,         mechanical and/or repulping treatment and has been subsequently         subjected to washing and optional oxygen delignification;     -   b. adjusting the pH of the alkaline cellulosic pulp so obtained         at least twice with carbon dioxide before a neutral treatment         and at least once with a strong acid before a subsequent acidic         treatment;     -   c. providing a (bi)carbonate removal step between the first and         the second carbon dioxide induced pH adjustment; and     -   d. processing the pulp after the acidic treatment in a pulp or         paper making process to form a dried product selected from pulp,         board and paper.

The invention is based on the realization that buffering (bi)carbonates can be removed from a pulp without at the same time significantly affecting the pH. Consequently, the invention relates to the use of (bi)carbonate removal for reducing the consumption of acid in the acidification of an alkaline cellulosic pulp.

In an embodiment of the invention, the pH of the pulp is reduced twice with carbon dioxide, and (bi)carbonate ions created in the first pH reduction are removed before the second pH reduction.

In a preferred embodiment of the invention, the pulp comprises a alkaline pulp produced in a pulp mill by chemical digestion. The pH reduction twice with carbon dioxide and the (bi)carbonate removal is followed by an acidification which comprises pH reduction of the (bi)carbonate containing pulp with a strong acid. The pH adjustment twice with carbon dioxide is preferably followed by an enzyme treatment and said acidification with said strong acid is typically followed by an acidic bleaching stage, such as chlorine dioxide bleaching.

In the present specification and claims, the terms generally have the meaning which is conventional in the art, e.g. as indicated in the hand book series called Papermaking Science and Technology, edited by J. Gullichsen and H. Paulapuro (ISBN 952-5216-00-4). Specifically, the following terms have the meanings defined below.

The term “pulp” and “cellulosic pulp” refers to a mixture or suspension of cellulosic pulp fibers in an aqueous medium. When reference is made to the specific aqueous medium within the fibers, the term used is “fiber water”. The pulp in the present description and claims has been produced by chemical or mechanical treatment of wood chips or by repulping of recycled fibers. The chemical treatment is a Kraft pulping process, a sulfite pulping process or any other chemical pulping process. The term mechanical pulping includes various forms of mechanical and also semi-mechanical processes. The amount of fibers calculated by weight basis (consistency) in the aqueous medium may vary from less than 1% to more than 50%. The pH and (bi)carbonate content of the cellulosic pulp is measured from the aqueous medium. The term “dried pulp” is used to refer to the end product made from the aqueous cellulosic pulp.

The term “alkaline cellulosic pulp” refers to a pulp suspension having a pH above 7. The alkaline cellulosic pulp has the pH resulting from the digestion, washing and/or oxygen delignification treatment to which it has been subjected. Typically this pH value is between pH 8 and 13.

The term “neutral treatment” refers to a treatment in a pulp and/or paper mill taking place at a pH close to 7. Such treatments include treatments such as enzyme treatments, chelation stages, flotation stages and refining procedures. Dithionite bleaching performed in the neutral pH range may also count as a neutral treatment. Typically the treatment is a chemical treatment performed at a pH between 6 and 9.

The term “acidic treatment” refers to a treatment in a pulp and/or paper mill taking place at a pH below 7. Typically the treatment is performed at a pH between 1 and 6. Such treatments include acidic bleaching treatments such as bleaching with elemental chlorine, chlorine dioxide, ozone, dithionite and peroxyacid; sizing with acidic sizing agents such as rosin and alun, etc.

The term “(bi)carbonate” refers to one or both of the ions CO₃ ²⁻ (carbonate) and HCO₃ ⁻ (bicarbonate). The ions are mostly present in the aqueous medium of the pulp. The two ions change from one to the other depending on pH and precipitation/dissolution of salts of said ions.

The term “(bi)carbonate removal” refers to a treatment of a pulp suspension which contains (bi)carbonate ions. In the (bi)carbonate removal, negatively charged (bi)carbonate ions are removed with counter cations other than the hydrogen ion (H⁺). A selective removal of (bi)carbonate ions is possible because of the affinity of the hydrogen ions to the fibers of the pulp. When (bi)carbonate ions are removed without removing corresponding hydrogen ions, the pH of the pulp is not affected, although the buffering capacity of the suspension is reduced.

The term “(bi)carbonate removal step” refers to a unit operation performed in a device of the pulp or paper mill. The unit operation may comprise a mere washing step (i.e. adding of water to the pulp and removal of water from the pulp) or it may comprise a washing step combined with a subsequent compression step. Individually, the washing and compression are referred to as “modes of (bi)carbonate removal”.

The term “carbon dioxide induced pH adjustment” refers to an addition of carbon dioxide directly or indirectly into the pulp in such a manner that the pH of the pulp is changed. When carbon dioxide is added to an alkaline pulp, the pH is reduced because the added carbon dioxide creates carbonic acid H₂CO₃ in the aqueous medium. The aqueous medium to which the carbon dioxide is added in the invention may be the aqueous medium of the pulp or it may be an aqueous medium, such as water, which is then added to the pulp.

The term “buffering capacity” refers to a measure of the resistance of a buffer solution to pH change on addition of hydroxide ions. A buffer solution is an aqueous solution of a mixture of a weak acid and its conjugate base. It has the property that the pH changes very little when a small amount of acid or base is added to it.

The following example is given to further illustrate the invention and is not intended to limit the scope thereof. Based on the above description a person skilled in the art will be able to modify the invention in many ways to derive the best advantages from the invention, for any practical purpose. The amounts of chemicals are calculated per metric tons dry pulp unless otherwise specified.

Example

A process according to FIG. 1 is operated in a Kraft pulp mill making softwood kraft pulp. The fiber line comprises a continuous digester 1, a washer 2, screening 3, a second washer 4, oxygen delignification 5, a first wash press 6, a second wash press 7, a storage tank 8, a tower 9 with enzyme (xylanase) treatment, and a chlorine dioxide bleaching 10, followed by optional extraction and further bleaching sequences (not shown).

The pulp comes out of the oxygen delignification tower at a pH of 11.5 and a consistency of 10%. Filtrate from second wash press 7 is used as wash water in first wash press 6. The washed pulp from wash press 6 is compressed to a consistency of 30%. The pulp is then diluted to a consistency of 6% with water and is fed to the second wash press 7, where it is washed with fresh water and again compressed to 30% consistency.

The enzyme treatment 9 requires a pH of 6.5. For the enzyme treatment the pH is adjusted by adding gaseous CO₂ into a pipe leading to the tower 9 so as to provide an ingoing pH of 6.5. The enzyme treated pulp is bleached with chlorine dioxide and its pH is adjusted before the ClO₂ stage with sulfuric acid to reach pH 3 out from the ClO₂ bleaching tower 10.

Taking the above process as reference (without admitting that the process as such forms part of the prior art), the procedure is changed to test the operation of the invention. For the test runs, CO₂ is added in amounts shown in Table 1 to the water used for diluting the pulp compressed to 30% consistency after first wash press 6. The CO₂ addition point is marked in FIG. 1 with a dotted line between wash presses 6 and 7. The pH of the pulp into the enzyme treatment and out from the bleaching tower, respectively, is maintained at the same level as in the reference (pH 6.5 and 3, respectively) by correspondingly reducing the amount of CO₂ into the enzyme treatment and the amount of H₂SO₄ into the ClO₂ treatment.

TABLE 1 pH from 2nd pH into enzyme pH out from wash press treatment ClO₂ tower Reference 10.5 6.5 3 Test 1 9.2 6.5 3 Test 2 9.3 6.5 3

The amount of (bi)carbonate removal in second wash press 7 in the test runs 1 to 3 was between 80 and 87%. The influence of the (bi)carbonate removal on the consumption of acid is indicated in Table 2.

TABLE 2 strong acid before CO₂ addition 1 CO₂ addition 2 ClO₂ kg CO₂/ton pulp kg CO₂/ton pulp kg H₂SO₄/ton pulp Reference 0 9 6.1 Test 1 5 3 2.8 Test 2 4 4 3.3 Test 3 2 7 5.0

As clearly seen in Table 2, the separate CO₂ induced pH adjustments with the (bi)carbonate removal between them reduces the amount of strong acid needed for adjusting the pH for the acidic treatment. Furthermore, the division of the CO₂ induced pH adjustment into two CO₂ additions may also be used to reduce the amount of CO₂ needed for the CO₂ induced pH adjustment.

The invention has been described herein with reference to specific embodiments. It is however clear to those skilled in the art that the process(es) may be varied within the bounds of the claims. 

1. A process for adjusting the pH of a cellulosic pulp, characterized in a) providing an alkaline cellulosic pulp, wherein said pulp has been produced by a chemical, mechanical or repulping treatment and has been subsequently subjected to washing and optional oxygen delignification; b) adjusting the pH of the alkaline cellulosic pulp so obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment; and c) providing a (bi)carbonate removal step between the first and the second carbon dioxide induced pH adjustment.
 2. The process according to claim 1, wherein said carbon dioxide induced pH adjustment(s) is/are performed by adding gaseous carbon dioxide to the pulp and/or to water used for diluting and/or washing the pulp.
 3. The process according to claim 1, wherein the first carbon dioxide induced pH adjustment is performed in connection with pulp washing.
 4. The process according to claim 1, wherein the second carbon dioxide induced pH adjustment is performed as a step separate from pulp washing prior to said neutral treatment to adjust the pH of said pulp to suit said neutral treatment.
 5. The process according to claim 1, wherein the pH adjustment with a strong acid is performed after said neutral treatment and before said acidic treatment and the pH of the pulp is adjusted with said strong acid to suit said acidic treatment.
 6. The process according to claim 5, wherein there is a second bicarbonate removal step after said neutral treatment and before said pH adjustment with a strong acid.
 7. The process according to claim 1, wherein said (bi)carbonate removal step comprises using water for displacing and/or washing (bi)carbonate ions from the pulp.
 8. The process according to claim 7, wherein said water comprises shower water applied to the pulp in a filter.
 9. The process according to claim 1, wherein said (bi)carbonate removal step comprises dewatering and/or compression of said pH-adjusted pulp and subsequent dilution with water.
 10. The process according to claim 7, wherein said water used for displacing, washing and/or dilution has a lower content of (bi)carbonate ions than the pulp being subjected to said (bi)carbonate removal step.
 11. The process according to claim 10, wherein said water is selected from fresh water, condensate, clear filtrate and washing water/filtrate from a downstream pulp washing.
 12. The process according to 1, wherein said (bi)carbonate removal step comprises at least two (bi)carbonate removal modes selected from washing, filtration, displacement, dewatering and compression of the pulp.
 13. The process according to claim 1, wherein said pulp comprises a Kraft pulp which has been subjected to oxygen delignification and said (bi)carbonate removal step comprises at least one step of washing the pulp.
 14. The process according to claim 1, wherein the initial pH of said alkaline cellulosic pulp is between pH 8 and 13, the pH is adjusted to between pH 7 and 12 in the first carbon dioxide induced pH adjustment, the pH is adjusted to between pH 6 and 9 in the second carbon dioxide induced pH adjustment and the pH is adjusted to between pH 2 and 7 with the strong acid.
 15. The process according to claim 1, wherein said strong acid is selected from the group consisting of sulfuric acid, sulfurous acid, bisulfite, chlorine dioxide, waste acid from chlorine dioxide production and alum.
 16. The process according to claim 1, wherein, said neutral treatment is selected from an enzyme stage, a chelating stage, a flotation stage, a refining stage and a dithionite bleaching stage.
 17. The process according to claim 1, wherein said neutral treatment comprises an enzyme treatment after the second pH adjustment with carbon dioxide.
 18. The process according to claim 1, wherein said acidic treatment comprises an acidic bleaching treatment with a bleaching chemical selected from chlorine dioxide, ozone, peroxyacid and dithionite.
 19. The process according to claim 1, wherein said (bi)carbonate removal step removes at least 25% of the (bi)carbonate ions created by the first carbon dioxide induced pH adjustment.
 20. A process for the production of pulp, board or paper from cellulosic pulp, characterized in a) providing an alkaline cellulosic pulp in a pulp and/or paper mill, wherein said pulp has been produced by a chemical, mechanical and/or repulping treatment and has been subsequently subjected to washing and optional oxygen delignification; b) adjusting the pH of the alkaline cellulosic pulp so obtained at least twice with carbon dioxide before a neutral treatment and at least once with a strong acid before a subsequent acidic treatment; c) providing a (bi)carbonate removal step between the first and the second carbon dioxide induced pH adjustment; and d) processing the pulp after the acidic treatment in a pulp or paper making process to form a dried product selected from pulp, board and paper.
 21. A process for reducing the consumption of acid in the acidification of an alkaline cellulosic pulp by (bi)carbonate removal.
 22. The process according to claim 21, wherein the pH of said pulp is reduced twice with carbon dioxide, and (bi)carbonate ions created in the first pH reduction are removed before the second pH reduction.
 23. The process according to claim 21, wherein said acidification comprises pH reduction of a (bi)carbonate containing pulp with a strong acid.
 24. The process according to claim 22, wherein said pH adjustment twice with carbon dioxide is followed by an enzyme treatment and said acidification with said acid is followed by acidic bleaching. 